This invention relates to a method of, and apparatus for, setting an operating of a voltage controllable optical components in communication systems. More especially, although not exclusively, the invention concerns optical modulators such as Mach-Zehnder interferometers and in particular such interferometers fabricated in Lithium Niobate.
A Lithium Niobate (LiNbO3) Mach-Zehnder Interferometer, hereafter known as an Mz device, is a voltage controlled optical attenuator having a optical attenuation/voltage characteristic which is periodic. Its use is primarily directed towards DWDM (dense wavelength division multiplexing) applications where collections of precisely controlled optical wavelengths (often termed wavelength channels) are transmitted through long-haul optical fibres. Such arrangements permit a single optical fibre to carry digital data at extremely high bandwidths. The MZ device, when inserted into the optical path at the output from the transmit laser, can be used to provide two differing functions:
i. To accurately control the optical power level of a particular wavelength.
ii. To modulate the optical signal with high-speed digital data.
From a control point of view, both functions are virtually identical. The only notable difference is that a MZ device used for modulation usually has two differential input signals. One carries the high-speed data while the other is used to set the bias point of the device. The present invention is relevant to both functions i and ii above. In the case of ii, however, it is the biasing aspect that is of particular interest.
FIG. 1 shows a block diagram illustrating the principle of operation of a MZ device. Light (optical radiation) enters the MZ device from an optical fibre cable and is split into two paths at S. Half the incident optical power passes through each path. Light passing along each path experiences a respective delay that is introduced by a slab of Lithium Niobate (LiNbO3). By applying an electric field, the result of external control voltages V1 and V2, to the LiNbO3 the delay can be varied. Light which has travelled along the two paths, each of which has experienced a potentially delay, is combined at C before being fed to the outgoing fibre cable.
It can be shown that ratio of output amplitude to input amplitude varies with the differential delay between the two paths. If this delay is τ and the angular frequency of the particular optical wavelength is ω then:Amplitude ratio=output amplitude/input amplitude=0.7071 (1+cos (ωτ))0.5
It follows that the corresponding power ratio (i.e. output power/input power) is a raised cosine response:power ratio=½(1+cos (ωτ))
If one control voltage is fixed and the other one varied V (ignoring imperfections and non-linearities) it can be shown that:Power ratio=½(1+cos (π(V−V0)/Vπ))
Where Vπ and V0 are constants. Vπ represents the voltage difference between successive peaks and troughs of output power and V0 represents a control voltage which results in maximum output power. Although V0 is multi-valued, the value nearest to zero volts is usually quoted.
FIG. 2 shows a graph illustrating the variation of power ratio with applied control voltage V; the V-P characteristic. It should be noted that the calibration of the Y-axis is linear with respect to power. It will be further appreciated that the variation of normalised optical attenuation versus control voltage (voltage/optical attenuation characteristic) is the reciprocal of that in FIG. 2 such that when the control voltage=V0, the power ratio is at a maximum value whilst optical attenuation is correspondingly at a minimum value.
As shown above, constants V0 and Vπ are the key to controlling an MZ device. Unfortunately, these so-called constants are in practice variables which depend upon:
i. Manufacturing tolerances (their post-manufacture values are difficult to control).
ii. Ageing (their values drift with age).
iii. Environmental conditions (such as temperature).
Any control circuit which is designed to maintain some specified output power level therefore needs to perform the following conceptual steps:                (a) Measure the power level at the output fibre (P).        (b) Compare (P) with the required value (which may be variable).        (c) Adjust the control voltage (V) so as to bring P towards the required power.        
Step (c) requires a knowledge of the sign of the “V-P characteristic” slope so that the feedback loop can apply an error correction signal which counteracts the unwanted change in output. Should the control voltage stray beyond the bounds of the chosen π segment (i.e. Vπ) of the characteristic then the control mechanism is likely to fail because positive feedback will then exist, leading to an increase in the error. Even a transient variation in the control voltage, caused by for example a noise transient, could be sufficient to do this. The control loop will then run away until the next π segment is reached, when negative feedback will again be restored. However, operation in a π segment remote from the π segment closest to V=0 means that the device is operated using an unnecessarily high voltage, leading to greater electrical strain, increased instability and possibly a shorter operating life. The only option would then be to set the control voltage V to some initial condition and then re-enable the control loop.
It is therefore necessary to calibrate the device to determine an acceptable initial condition for the control voltage V which would guarantee starting the control loop within the chosen π segment of the characteristic and which is suitably clear of the turning points of the V-P (voltage/optical attenuation) characteristic.